fioi

[Content by Gemini 2.5]

Ransomware Brief – “.fioi” Variant (part of the STOP/DJVU family)

Technical Breakdown

  1. File Extension & Renaming Patterns
  • Confirmed extension: .fioi (lower-case, appended after the original extension).
  • Typical renaming pattern: <original_name>.<original_ext>.fioi
    Example: Annual_Report.xlsx ➔ Annual_Report.xlsx.fioi
  • Note: System/boot files are skipped; focus is on user documents, pictures, archives, PST/OST files, etc.
  1. Detection & Outbreak Timeline
  • First submitted to ID-Ransomware / VirusTotal: mid-October 2023.
  • Rapid spike observed: November-December 2023 (coincided with malvertising campaigns pushing fake software installers).
  • Still circulating heavily through crack, keygen, and “free software” sites as of Q2-2024.
  1. Primary Attack Vectors
  • Malvertising → fake software installers (64-bit NSIS or MSI bundles):
    – Top lures: “MS Office 2023 crack”, “Adobe Photoshop 2024 pre-activated”, “Windows 11 activator”.
  • Bundled in “drive-by” updates from P2W (pay-per-install) networks: user thinks they’re getting a game mod; drops .fioi.
  • RDP / SMB brute-forcing observed in a minority of cases—usually precursor to manual deployment of multiple malware families (Raccoon, Vidar, then .fioi).
  • No current signs of worm-like spread (EternalBlue, BlueKeep) for .fioi; infection tends to be local to the clicked-on machine.

Remediation & Recovery Strategies

  1. Prevention (STOP/DJVU-specific)
  • Strip local admin rights from daily-use accounts.
  • Disable Office macro execution via GPO unless business-critical.
  • Use reputation-based web controls to block “keygen” & “warez” categories—#1 entry point.
  • Patch publicly exposed RDP; enable NLA, lock to 2–3 attempts before IP ban (e.g., Windows Account Lockout + RDPGuard).
  • Deploy application whitelisting (Windows Defender Application Control or AppLocker) to block %LOCALAPPDATA%\Temp\*.exe launched by MSI/NSIS.
  • Keep offline backups: STOP variants skip mapped drives that show as “removable” but will encrypt NAS volumes visible as regular drive letters.

  1. Removal (step-by-step)

  2. Physically disconnect the machine from network/Internet.

  3. Boot into Safe Mode with Networking.

  4. Use a clean PC to download:
    – ESET Online Scanner or Malwarebytes (latest)
    – “Michael Gillespie’s STOP-Decrypter-support” bundle (for later key testing).

  5. Install and update the AV, run full scan; quarantine all items tagged Trojan:Win32/Stop.R or Variant.

  6. Delete scheduled tasks under \Microsoft\Windows\ named randomly (e.g., “updatesys”, “service64”).

  7. Inspect registry Run keys (HKCU\Software\Microsoft\Windows\CurrentVersion\Run) for executable inside %USERPROFILE%\AppData\Local\ with random 4-letter name; remove value.

  8. Empty temp folders, restart normally.

  9. Patch exposed services, reset all local/AD passwords (offline) in case infostealers dropped with ransomware.

  10. File Decryption & Recovery

  • Feasibility: Mixed.
    • If the malware contacted its C2 and fetched a unique online key ➔ files are NOT decryptable without that key.
    • If the victim PC was offline or the C2 was down, the malware falls back to a hard-coded offline key ➔ decryption IS possible using the STOP family decryptor.
  • Tool: Emsisoft Stop/Djvu Decryptor (free, kept current).
    – Launch as administrator.
    – Point it at a PAIR of original+encrypted files (readme.txt + readme.txt.fioi) to test offline key validity.
    – If “Decryption key is present” appears, let the tool run across the whole drive.
  • Data-recovery alternatives (when encryption key unknown):
    – Try Windows “Previous Versions”/ShadowCopy; newer STOP versions delete shadows, but not always.
    – Recycle-bin data recovery tools may restore original copies of small documents the ransomware auto-deleted after encryption.
    – Paying the ransom ($490–$980) is discouraged: support is poor, many victims receive only partial keys.
  1. Other Critical Information
  • Doppelgänger payload: .fioi installers frequently drop the Vidar or RedLine info-stealer BEFORE encryption, so assume credentials, browser cookies, and crypto-wallet files are already exfiltrated. Force password resets and enable 2FA.
  • Persistent schedule: Creates a scheduled task that respawns the payload if admin tries to kill it; removal in Safe Mode is therefore essential.
  • Ransom note filename: _readme.txt (same across STOP variants), placed in every folder.
  • No evidence that .fioi manipulates partition tables or Master Boot Record; system will boot normally after cleaning.
  • Wider impact: Because cracks remain publicly available on high-traffic “abandon-ware” forums, .fioi is one of the most submitted STOP variants to ID-Ransomware in 2024, disproportionately hitting SOHO users in Europe & South America.

Bottom line: Remove the malware immediately, test Emsisoft’s decryptor with an original/encrypted file pair, rotate all credentials, and move to 3-2-1 (offline) backups to outsmart future campaigns.